A blowout preventer is basically a pressure vessel with moving parts, both inside and out. A key portion of the pressure vessel is the body. The body must have sufficient strength to contain pressured well fluids, (as a pressure vessel), and supply a guide and sealing means for the moving internal parts.
These two functions are contradictory in material requirements. To serve as a pressure vessel, the body and ram block material must meet certain specifications to resist stress cracking in the presence of such chemicals as hydrogen sulphide. This typically means that the material must be relatively soft. Unfortunately, this soft material must also provide a guide and seal means for the moving ram blocks in the extremely abrasive environment of the well fluid. Solving the deterioration of the contacting surfaces would normally require harder, more abrasive resistant materials. These materials however, are either expensive or do not meet the specifications of the soft materials to meet the requirements to resists stress cracking.
The normal practice is to manufacture the body, (pressure vessel), and the ram blocks from softer materials to meet stress cracking specifications, and repair all parts when necessary as they wear. The repair of the material of these components requires a specific weld procedure that is expensive. In addition, the weld material must also be soft and wears as readily as the original material.
Typically, the body is weld repaired several times over without regard to the consequences to the parent material when subjected to the numerous heat cycles involved. A weld repair is a fusion process that involves heat applied to the parent material in excess of it's′ “critical” temperature to fuse with the added weld material to the parent material. Repeated cycles of this nature can permanently change the structure of the parent material and lead to a host of other problems.
There are a number of solutions that have been attempted, but for various reasons, have been unable to solve the soft and hard material preferences for this application. One solution was to weld apply hard surfacing to the parent material where the moving ram blocks contacted the body. This process was unsuccessful due to the expense and inability to meet the material requirements to resist cracking. Because hard surfacing is a fusion process, the hard material becomes a part of the body and must meet the same property specifications to resist cracking. Unfortunately hard surfacing materials would not pass some specific requirements, usually associated with crack initiation. With fusion processes, cracks that initiate in the hard surfacing may propagate through to the parent material, the body, which is to be protected from cracks. Another process that has been attempted to improve the wear of the mating surfaces is an infusion process that relies on a heated gas process to harden a thin layer of the parent material of the body. As in welding hard surfacing material to the parent material, the infusion process also becomes part of the body, or pressure vessel, and must meet all of the required specifications. Testing suggests that not only does the hardened layer not meet specifications, but also suggests that cracks could be initiated at lower stresses than the parent material. Again, crack propagation into the parent material from the hardened surface is of major concern for a pressure vessel.
A more effective system has been used that provides a replaceable seal seat. It is locked into position with a sprung ring and setscrew arrangement that locks an “o-ring” sealed seat into position. The major problem with this arrangement is that it often corrodes and may be extremely difficult to remove.